Multiplexers are one of the most commonly used functions in microprocessors and other integrated circuits. Multiplexers can be implemented in a large number of circuit families, each with various topologies. Dynamic multiplexers are generally used where high performance is critical.
A typical dynamic multiplexer is illustrated in FIG. 1, where s(0 . . . 3) are the pre-decoded select signals and d(0 . . . 3) are the corresponding data signals. This prior art dynamic/domino multiplexer circuit includes an NFET "foot" device (where the gate is connected to clock (clk)) in series with an NFET for select and another NFET for data. This results in a three-high n-stack (note that the "foot" device performs no logical function).
Such a "foot" device is implemented to eliminate "crowbar current" from occurring in such dynamic circuits. Crowbar current occurs when there is no foot device activated by the clock, and current is able to shoot down through the n-stack from the precharge node during the transition between the precharge and the evaluate phases.
Nevertheless, such a foot device introduces complexity into the circuit along with an increase in the height of the n-stack, a loading upon the clock signal, and an increase in area and power consumption of the circuit. As a result, there is a need in the art for a multiplexer that does not require such a foot device yet does not suffer from crowbar current.